Radiative cooling features the release of heat from an object or surface in the form of thermal radiation, thereby lowering the temperature of the object or surface, or maintaining its temperature at a relatively lower baseline when operating in steady state. The thermal radiation emitted from a non-zero-temperature surface (in Kelvin) has a continuous wavelength or frequency spectrum that depends on the temperature of the surface; most of the thermal radiation emitted by objects near room temperature is in the infrared region(s) of the electromagnetic spectrum.
A wavelength-selective radiative cooling device can emit thermal radiation preferentially in the selected wavelength ranges of the electromagnetic spectrum. For example, a selective radiative cooling device may preferentially emit infrared radiation at wavelengths corresponding to a “atmosphere window”, also known as an “atmospheric transmission window” or an “atmospheric transparent window”. The Earth's atmosphere is practically non-absorbing in these “window” wavelength ranges, such as at 7-13 μm and 16-30 μm. Examples of radiative cooling devices for emission of infrared radiation are described in U.S. Pat. Nos. 2,289,809; 3,043,112; 3,671,286; 4,586,350; and 7,503,971 and U.S. Patent Application Publications US2010/0155043 and US2015/0131023.